Certain luminescent substances (phosphors), comprising an inorganic compound (matrix) doped with ions of elements (activators), show a direct light emission upon exposure to ionizing radiation, in particular to X-rays or cathodic rays. These luminescent substances are commonly called direct emission phosphors. Examples of such phosphors are BaSO.sub.4 :Pb, Ba.sub.1-x Sr.sub.x SO.sub.4 :Eu, Gd.sub.2 O.sub.2 S:Tb and the like. The direct emission process of such phosphors can be explained by the fact that, after absorption of the radiation, an electron of the matrix crystal lattice is elevated to the conduction band from the valence stable band and thereafter falls down into the valence stable band; the recombination energy is then transferred to the nearest activator ion which is thus brought to an excited level; by the ion's return to the stable state, the excited ion produces a light emission characteristic of that element.
It is also known that some other matrices doped by ions of some elements show a light emission if photostimulated after exposure thereof to X-rays or Cathodic rays. These photoluminescent substances are commonly called photostimulable or storage phosphors. Examples of such stimulable phosphors are LaOBr:Ce,Tb and BaFCl:Eu. The photostimulated emission process is in this case explained by the presence of traps in the crystal lattice, in general F.sup.+ centers, i.e. vacancies of oxygen and/or bromine and fluorine and/or chlorine lacking in an electron; these centers trap the electrons produced upon exposure to the ionizing radiation and release them upon subsequent photostimulation; the released electrons are transferred to the activator element which generates its own characteristic light emission by the same mechanism as direct emission phosphors. A photostimulated luminescent mechanism of this type is for instance reported for BaFX:Eu in Journal of Luminescence 31 and 32 (1984), 266-268.
Among the photostimulable phosphors, in Phys. Status Solidi A 83, (1984), 597-604 there are described Eu and Sm doped calcium sulfates which, after exposure to X-rays or UV-rays, have a photostimulated luminescence characteristic of the activator element.
Generally, the photostimulable phosphors exposed to ionizing radiation show a direct luminescence also due to the activator elements. In these phosphors, therefore, both mechanisms are in reciprocal competition: the direct luminescence due to the excited activator elements and the photostimulated luminescence due to the storage of the exciting energy in suitable centers (traps) which upon subsequent photostimulation are capable of supplying electrons to excite such activators.
The photostimulated luminescence phosphors find a particular application in the methods for recording and reproducing an ionizing radiation image (e.g. digital radiography), where the phosphor which has absorbed the ionizing radiation is stimulated with light radiation to release the stored radiation energy in the form of fluorescent light.
In practice, such phosphors have some disadvantages, such as for instance a sensitivity quite insufficient for an industrial method, a persistence of the stored information which tends to decrease over time and, above all, the wavelengths of the fluorescent photostimulated light partially overlapping the stimulating radiation, thus making the use of optical filters necessary to separate such light from such radiation.
Therefore, from the point of view of the practical use of such methods, there is a need to have photostimulated luminescence phosphors with improved characteristics with respect to those phosphors known in the art, and most importantly photostimulated luminescence phosphors emitting fluorescent light in a region of the electromagnetic spectrum which is separated from the stimulating radiation region of the electromagnetic spectrum.
U.S. Pat. No. 3,859,527 describes a method for producing an image corresponding to a releasably stored image of pattern of high energy radiation comprising releasing the stored image as modulated light, converting the modulated light into corresponding electrical signal, producing intensity modulated light from the electrical signal and possibly recording the produced modulated light.
U.S. Pat. No. 4,236,078 describes a method for recording and reproducing a radiation image comprising exposing a stimulable phosphor with radiation passing through an object, stimulating said phosphor with stimulating visible or infrared rays and detecting the released fluorescent light, the stimulating rays having a wavelength of no less than 500 nm and the phosphor being chosen within the group including Eu and Pb activated zinc sulfide; Eu activated barium aluminate; Ce, Tb, Eu, Tm, Pb, Ti, Bi and Mn activated silicate of Mg, Ca, Sr, Zn and Ba; and Ce, Tb activated oxyhalides of La, Y, Gd and Lu.
U.S. Pat. No. 4,258,264 describes a method of reading out a radiation image stored in a stimulable phosphor comprising stimulating the phosphor with rays having a wavelength within a range of 600 to 700 nm and detecting the emitted light whose wavelength is within the range of 300 to 500 nm.